Certain geographical areas have large deposits of coal. However, the coal may be low-rank coal requiring a beneficiation, namely, to remove moisture and impurities and thus improve the BTU to weight ratio; and for this reason, the coal is treated in coal processing systems. In these systems, the coal is conveyed into (and out of) a pressure chamber having a controlled gaseous composition, wherein the coal is subjected to increased temperatures and pressures.
Rotary airlocks are an important part of the coal processing system 10 (as shown in FIG. 1). These rotary airlocks 11 and 11' are installed at the entrance and exit, respectively, of the pressure chamber (or fluidized bed) 12 for transferring the coal between successive processing operations, maintaining the pressure and temperature differential therebetween, and keeping gaseous compositions within the pressure chamber 12 and may include the airlock 11' with controlled feed rate and the free flow airlocks 11. Thus, the rotary airlocks are a major, and important, components in coal processing systems.
Unfortunately, however, the conventional rotary airlocks currently being used (in coal processing systems) tend to jam. The jamming is caused by hard materials found in the coal being processed, as for example, lumps of solid rocks which become stuck between the rotating vanes and the stationary walls of the rotary airlocks. When jamming occurs, the entire continuous coal processing system must be stopped in order to clear or unclog the jammed airlock. As somewhat schematically shown in FIG. 2, cleaning of the jammed airlock usually is a manual, time consuming and expensive operation; and permanent damage to the rotary airlock and its motor and drive systems may occur.
Although the prior art of rotary airlocks is well developed, nevertheless, all of these prior art rotary airlocks are subject to jamming or clogging.
For instance, U.S. Pat. No. 4,076,150 describes a rotary airlock with blades adjustable in such a manner so as to maintain the pressure seal. U.S. Pat. Nos. 4,750,273, 4,599,809 and 5,165,434 describe rotary airlocks powered by an electric motor (schematically shown in FIG. 3). U.S. Pat. Nos. 5,122,259 and 5,178,733 teach a rotary airlock with means for indicating and controlling the speed of rotation. However, none of these prior art patent references is concerned with preventing jammings of the airlocks.
In an effort to solve this problem, mechanical sensors have been suggested in the field to detect jams and, once detected, mechanical switches provide for a reverse rotation to clean the rotary airlock. Disadvantageously, the mechanical switches are unable to quickly sense jams and to take corrective actions in order to adequately prevent solid jams.
Therefore, a more reliable and less expensive means for quickly detecting when a rotary airlock (in a coal processing system) may jam, and for quickly preventing the jam and unclogging the rotary airlock, would be very desirable.